The majority of present day integrated circuits (ICs) are implemented by using a plurality of interconnected field effect transistors (FETs), also called metal oxide semiconductor field effect transistors (MOSFETs). A MOSFET device includes a gate structure that is interposed between a pair of spaced apart source and drain electrodes formed in and/or on the semiconductor substrate. The gate structure includes a gate electrode that is formed overlying a gate oxide layer, which serves as insulator between the gate electrode and a channel defined between the source and drain electrodes. The length of the channel is equal to the distance between the source and drain electrodes. The gate electrode serves as a control electrode that controls the flow of a current through the channel based on a control voltage (VG) applied to the gate electrode. When the control voltage (VG) applied to the gate electrode exceeds a threshold voltage (Vth) an electrical field is generated that causes a drive current to flow along the channel between the source and drain electrodes.
In many conventional MOSFETs, the gate electrode is formed of polysilicon and the gate oxide layer is formed of silicon dioxide (SiO2). Although highly-doped polycrystalline silicon is an acceptable gate electrode material, it is well-known that polysilicon is not an ideal conductor since it is much more resistive than metal, which reduces the signal propagation speed through the material. It is desirable that the gate oxide layer be made thin to increase the channel conductivity and performance when the transistor is on and to reduce sub-threshold leakage when the transistor is off. As device dimensions continue to shrink in an effort to pack more devices within a given layout area, the dimensions of the gate structure also shrink. One challenge that has arisen with the continued scaling of MOSFET devices relates to limitations on the thickness of the gate oxide layer particularly when it is made from SiO2. For example, as thickness of the SiO2 gate oxide layer decreases, gate oxide leakage increases. As devices are made smaller and thickness of the gate oxide layer decreases, carriers will eventually tunnel from the channel through the gate oxide layer and to the gate electrode. Thus, as new generations of integrated circuits and the MOSFETs that are used to implement those ICs are designed, technologists must rely heavily on non-conventional elements to boost device performance. Examples of such non-conventional elements include gate structures constructed using “high-dielectric constant (k) gate dielectrics” with “metal” gate electrodes.
High-k materials generally refer to materials with a dielectric constant (k) greater than that of silicon dioxide. In general, the dielectric constant (k) of a particular material is a measure of how much charge the particular material can hold. Capacitance of a material is proportional to its dielectric constant (k) and inversely proportional to dielectric thickness. Increasing the dielectric constant (k) of the gate dielectric allows for a thicker layer of material to be used while maintaining the same capacitance yet improving resistance to leakage currents. Therefore use of a higher dielectric constant (k) material can increase capacitance of the transistor, which improves the transistor's performance. Thus, as an alternative to the conventional SiO2 gate oxide layer, high-k gate dielectrics can be used to form the gate oxide layer of a transistor. A material with a higher dielectric constant allows the gate insulator to be made thicker than when a conventional SiO2 gate oxide layer is used, and a thicker gate insulator tends to reduce the resulting leakage current. Examples of common “high-k” materials include hafnium dioxide (HfO2), zirconium dioxide (ZrO2) and titanium dioxide (TiO2), and group IVb metal silicates such as hafnium and zirconium silicates.
Although a conventional polysilicon gate electrode can be used in conjunction with a high-k dielectric gate oxide, the combination of a high-k dielectric and a polysilicon gate electrode is not suitable for high performance logic applications. The resulting high-k/polysilicon transistors have high threshold voltages and degraded channel mobility, and hence poor drive current performance. Thus, instead of using polysilicon gate electrodes in conjunction with high-k dielectrics, metal gate electrodes, such as tantalum, tungsten, tantalum nitride, and titanium nitride, can be used to screen surface phonon scattering in high-k dielectric material and thereby improve channel mobility. The particular metal used to form the gate electrode varies depending on the particular high-k dielectric that is used for the gate oxide layer. The particular metal or metals selected for the metal gate electrode should have an appropriate work function to provide a desired threshold voltage, alleviate mobility degradation problems, and enable high-performance high-k/metal gate transistors with low gate dielectric leakages.
One issue that can arise with the use of high-k dielectric gate oxides relates to altering or changing the threshold voltage (Vth) of the MOSFET device. For example, oxygen vacancy sites within a high-k dielectric gate oxide can act as fixed positive charges that alter the electrostatic potential at the surface of the silicon layer that defines transistor channel. These fixed positive charges affect the threshold voltage (Vth) of the MOSFET device. For instance, in a PMOSFET device, the presence of the fixed positive charges (oxygen vacancy sites), requires that a greater negative gate voltage (−VG) be applied to the metal gate electrode to attract holes and create a channel in which majority carrier holes can flow. In other words, the oxygen vacancy sites effectively increase the threshold voltage (Vth) of the PMOSFET device (i.e., make it more negative).
To decrease the threshold voltage (Vth) of the PMOSFET, “lateral oxidation” techniques can be used to laterally diffuse oxygen, which is present near the edges of the gate electrode, along the channel underneath the gate electrode. The diffused oxygen fills oxygen vacancy sites thereby eliminating them. Since less oxygen vacancy sites or “fixed positive charges” remain along the channel, less gate voltage (VG) has to be applied to the gate electrode to offset any remaining fixed positive charges thereby lowering the threshold voltage (Vth) (i.e., making the threshold voltage (Vth) more positive).
It is desirable to provide an improved MOSFET devices which include gate structures constructed using a high-dielectric constant (k) gate dielectric with a metal gate electrode and methods for fabricating such MOSFET devices to have appropriate threshold voltages. For example, it is desirable to provide such an improved MOSFET device in which the number of oxygen vacancy sites is reduced and in which the distribution of oxygen vacancy sites is substantially uniform along the entire length of the channel. Furthermore, other desirable features and characteristics of the present invention will become apparent from the detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.